The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a fail-safe system for a failure of a sensor such as a throttle position sensor of an automotive engine.
A known continuously variable belt-drive transmission for a motor vehicle comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulley in dependence on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to servo device, and a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to set the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
In such a system, if a load detecting sensor such as a throttle position sensor is out of order and operates an erroneous signal, the transmission ratio control valve and the line pressure control valve malfunction due to the erroneous signal. Accordingly, the transmission ratio and line pressure deviate from desired values respectively, which will render the vehicle inoperative.
Japanese Patent Application Laid Open 59-77157 discloses a fail-safe system for a continuously variable transmission. The system is adapted to provide a maximum transmission ratio when an electromagnetic clutch for transmitting the output of the engine to the transmission is disengaged. Accordingly, even if the throttle position sensor fails to sense the load on the engine, the vehicle can be started at the maximum transmission ratio. However, the transmission ratio is not properly controlled after the starting of the vehicle.
On the other hand, the throttle position sensor may malfunction to generate an erroneous signal representing the wide-open throttle or the closed throttle. In case of the wide-open throttle signal, the transmission ratio changes along a line l, of FIG. 5. As seen from the figure, the ratio varies in a high engine speed range. To the contrary, in case of a closed throttle signal, the transmission ratio changes along a line l.sub.2 in a low engine speed range. Accordingly, if, for example, the engine is accelerated on an uphill road, the transmission ratio is up-shifted along the line l.sub.2 in the low engine speed range. In such a driving condition, since the driving torque of the engine is very low, it is difficult to drive the vehicle.